Mass spectral imaging (MSI) has become an important analytical technique that has been broadly utilized within a number of fields. Its utilization is prominent in materials analysis and it has been utilized for diverse applications from metals characterization to biochemistry. It has been growing in importance, especially for the analysis of tissues and other biological samples. By generating an analyte map of a surface, valuable information about how a certain organism uses a given analyte can be obtained. Conventionally, MSI is performed with an ionization source that is under vacuum, such as matrix-assisted laser desorption/ionization (MALDI) or secondary ion mass spectrometry (SIMS). McDonnell, L. A. Heeren, R. M. Mass Spectrom. Rev. 2007, 26, 606-643. To date, two methods for performing MSI with MALDI have been demonstrated. The most common technique, termed MALDI probe imaging, scans a pulsed UV laser across a sample surface that is evenly coated with a UV-absorbing matrix. Caprioli, R. M.; Farmer, T. B.; Gile, J. Anal. Chem. 1997, 69, 4751-4760. The resulting time-trace can be converted into a distance plot and the chemical image can be compiled. An alternative method, termed MALDI microscope imaging, pulses a defocused UV laser spot onto a matrix-covered sample to envelop a large area. Luxembourg, S. L.; Mize, T. H.; McDonnell, L. A.; Heeren, R. M. A. Anal. Chem. 2004, 76, 5339-5344. Special ion optics preserve and magnify the shape of the resulting ion packet. The ions are then detected with an intensified charged coupled device (iCCD) after traveling through a conventional time-of-flight (TOF) mass analyzer for mass-to-charge (m/z) separation. This technique may require specialized ion optics, fast electronics/detectors, and complex computing to regenerate chemical images. Correspondingly, it may be seen that few m/z (mass-to-charge ratio) values can be detected with each run due to the detector response time and the size of the data files generated. Klinkert, I.; McDonnell, L. A.; Luxembourg, S. L.; Altelaar, A. F. M.; Amstalden, E. R.; Piersma, S. R.; Heeren, R. M. A. Rev. Sci. Instrum. 2007, 78. There are numerous other potential challenges in using MALDI for MSI. One potential challenge is that samples are analyzed under vacuum, potentially presenting the additional steps of drying and mounting a biological, or wet, sample prior to analysis. Another potential challenge is that the matrix solution must be applied to the sample evenly and in small enough droplets to obtain high spatial resolution while maintaining an optimal matrix to analyte ratio.
A class of atmospheric-pressure ionization sources for mass spectrometry, collectively termed ambient mass spectrometry (AMS), have been developed and shown to be well suited for MSI. One example of sampling at atmospheric-pressure includes a recently developed infrared MALDI (AP-IR-MALDI) for MSI. Li, Y.; Shrestha, B.; Vertes, A. Anal. Chem. 2007, 79, 523-532. In this case, an IR laser tuned to a vibrational band of water was used for the desorption/ionization process, so that water would act as the matrix. A potential limitation of the AP-IR-MALDI technique was the diffraction limit of the laser (˜250 micrometer (μm)). While the technique did result in chemical images, challenges persist in obtaining improved signal-to-noise ratios.
Another method for MSI of an atmospheric-pressure sample is the use of desorption electrospray ionization (DESI). Wiseman, J. M.; Ifa, D. R.; Song, Q. Y.; Cooks, R. G. Angewandte Chemie-International Edition 2006, 45, 7188-7192. DESI uses a high velocity gas stream to impact solvent droplets from an electrospray ionization (ESI) source onto a surface. Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science (Washington, D.C., United States) 2004, 306, 471-473. When the large solvent droplets impact the sample surface, they break up into smaller droplets and pick up analyte molecules from the surface. The smaller droplets, which have a scatter angle of ˜10°, are drawn into the capillary interface of a mass spectrometer. Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M. Science 2006, 311, 1566-1570. Costa, A. B.; Cooks, R. G. Chemical Communications 2007, 3915-3917. Takats, Z.; Wiseman, J. M.; Cooks, R. G. Journal of Mass Spectrometry 2005, 40, 1261-1275. Venter, A.; Sojka, P. E.; Cooks, R. G. Anal. Chem. 2006, ACS ASAP. As the analyte-containing solvent droplets evaporate, the charge originally on the droplet is transferred to the analyte molecule. To perform MSI, the sample stage is scanned underneath the fixed-position DESI tip and mass spectrometer capillary interface. Spatial resolutions may be limited by droplet scatter, sample smearing, and DESI tip diameter (˜200 micrometer (μm)). Ifa, D. R.; Gumaelius, L. M.; Eberlin, L. S.; Manicke, N. E.; Cooks, R. G. Analyst 2007, 132, 461-467. Both of these techniques have the advantage that high-mass molecules can be detected; however the spatial resolution is still an aspect which could be improved.
Another technique, termed laser ablation electrospray ionization (LAESI), requires no sample pretreatment, can operate at atmospheric-pressure, and offers the potential of depth information. In this technique, laser ablation using a mid-IR laser removes material from a surface and ESI is used to directly ionize molecules from the ablation plume. By coupling laser ablation with ESI, good detection limits have been achieved, 5 fmol for verapamil, while maintaining a broad detectable mass range (up to 66 kDa). Nemes, P.; Vertes, A. Anal. Chem. 2007, 79, 8098-8106.